1. Technical Field
The present disclosure relates to laser apparatuses.
2. Related Art
Recently, the miniaturization and increased levels of integration of semiconductor integrated circuits have led to a demand for increase in the resolution of semiconductor exposure apparatuses (hereinafter, referred to as “exposure apparatuses”). Accordingly, advances are being made in the reduction of the wavelengths of light outputted from exposure light sources. In general, gas laser apparatuses are being used as exposure light sources instead of conventional mercury lamps. For example, a KrF excimer laser apparatus that outputs an ultraviolet laser beam having a wavelength of 248 nm and an ArF excimer laser apparatus that outputs an ultraviolet laser beam having a wavelength of 193 nm are used as gas laser apparatuses for exposure.
Immersion exposure, in which a gap between an exposure lens of an exposure apparatus and a wafer is filled with a liquid, has been put into practical use as a next-generation exposure technique. With the immersion exposure, the apparent wavelength of an exposure light source is reduced since the refractive index between the exposure lens and the wafer is changed. In the case where immersion exposure is carried out using an ArF excimer laser apparatus as the exposure light source, the wafer is irradiated with ultraviolet light having a wavelength of 134 nm within the liquid. This technique is called “ArF immersion exposure” (or “ArF immersion lithography”).
The spontaneous oscillation widths of a KrF excimer laser apparatus and an ArF excimer laser apparatus are relatively wide, such as between approximately 350 to 400 pm. Therefore, if a projection lens is made of such a material that transmits ultraviolet light such as KrF and ArF laser beams, chromatic aberration occurs in some case, which can cause the resolution to drop. Accordingly, it is necessary to narrow the spectral bandwidth of the laser beam outputted from the gas laser apparatus until the chromatic aberration becomes small enough to be ignored. For this reason, a line narrow module having a line narrowing element (an etalon, a grating or the like) is provided within a laser resonator of the gas laser apparatus so as to narrow the spectral bandwidth in some case. Hereinafter, a laser apparatus in which the spectral bandwidth is narrowed is called a line narrowing laser apparatus.